Error for autodoc on empty module

[krystophny]

Running sphinx with fortran_autodoc on an empty module fails at

parsing fortran sources...{}

Exception occurred:
  File ".../sphinx-fortran/sphinxfortran/fortran_autodoc.py", line 347, in scan
    m = block['vardescsearch'](line)
KeyError: 'vardescsearch'

Quick fix: add a check if 'vardescsearch' in block.keys(): before line 347 in fortran_autodoc.py. A better solution would be to unify the initialization of block for all cases.

[krystophny]

See https://github.com/itpplasma/sphinx-fortran/commit/9568689bad1089bcb69a01f793daabbaead617e3